Avalanche Forecast

Issued: Mar 25th, 2019 11:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

A weak storm system impacting the area Monday night and Tuesday morning may create new avalanche concerns at higher elevations. Be on the lookout for places where the wind drifted the new snow into slightly firmer and thicker slabs. When you see wind transported snow, be leery of nearby slopes greater than 35 degrees.

Summary

Discussion

Snow and Avalanche Discussion

This storm system doesn’t appear to be a big water producer, but it will be enough to change the conversation around our primary avalanche problem. Cooler temperatures should help the l snowpack to begin to freeze and gain strength at all elevations. This will shift our focus to the new snow. Just because we are forecasting “low” avalanche danger below treeline, doesn’t mean you can’t find isolated areas of unstable snow. Primarily, you want to watch for areas of unconsolidated wet snow where you may trigger a lingering loose wet avalanche. Even small avalanches can harm you if they push you into a creek or gully.

You may find a few sun-breaks especially later in the day around the West-North zone. It’s March, so if the sun does make an appearance, you’ll see snow conditions change rapidly. Expect natural small loose wet avalanches to occur on steep sunny slopes. Don’t let this easy to predict and avoid avalanche concern catch you off guard.

Even with cooling temperatures, the snowpack continues to undergo a spring transition. As a result, you may encounter a wide variety of snow conditions including firm icy surfaces, breakable crust, wet heavy snow, and shallow powder. Be on the lookout for opening creeks and glide cracks. Use caution if you travel near these features.

Snowpack Discussion

March 22nd, 2019

Enter Spring

If you’ve been in the snow recently, the wintery conditions of early March may seem worlds away. You may be in for a surprise if it’s been a while since you were in the mountains. The weather has taken a turn towards spring in the last couple weeks and the Cascade snowpack the has undergone major changes. Unseasonably warm temperatures and strong sun followed a month-and-a-half of cold, winter storms. Mid-elevation weather stations stayed above freezing from March 15th-22nd with high temperatures reaching the upper 50’s to low 60’s. For an in-depth survey of the regional snowpack, we’ll divide the terrain up by aspect and elevation.

A graph showing temperatures between 4,000-5,000ft around the Cascades from the 16th-21st.

Aspects

Northerlies

Along with the warm temperatures, the spring sun has played a major role in warming snow surfaces. The result is a snowpack that varies by aspect. In most regions, shaded and northerly slopes remain relatively unchanged. Aside from some settlement and firmer or moist surfaces, the snow on north aspects is almost entirely dry. Even some low elevation north slopes are still holding snow.

Sunny slopes

The snowpack on east through south through west aspects is a different story. The strong March sun melted snow surfaces and drove melt-water into the snowpack. This is most dramatic on steep (over 35 degrees) southeast through southwest slopes below 5,000ft.  In some areas, you can find meltwater up to 3 feet below the snow surface with drainage channels well established. Between this warm period and rain events in the first half of the winter, the entire snowpack has transformed to melt forms. An important point to note is that as of the 22nd, these solar aspects remain unfrozen and weak. Cooler weather ahead may help strengthen moist to wet layers.

A glide avalanche (D2) released from a rock slab late on the 20th. Lichtenberg Mtn, 5,100ft, SE aspect. Other glide avalanches occurred on the 20th at Snoqualmie Pass and in Tumwater Canyon. Photo: Josh Hirshberg

Elevation

Low elevations

As you travel from low valleys to higher peaks, you’ll notice a major difference in the snowpack based on elevation. With all the low-elevation snow this winter, there are still some cold, shaded slopes holding pockets of snow down to 1,000ft, especially east of the Cascade Crest. However, most slopes below 3,000ft have lost much of their snow cover. Many low elevation, sun-exposed slopes are bare, especially in areas that previously held less than 3 feet of snow. The low elevation snowpack is no longer substantial enough to allow for easy travel over snow or widespread avalanches.

Loose wet avalanches on the south side of Table Mtn, near Mt Baker. 3/17. Photo: Pete Durr

Mid-elevations

At mid-elevations, around 3,000-5,000ft, the snowpack is still deep and layered. Many slopes at this elevation band near and west of the Cascade Crest are holding 6-10 feet of snow. This is also where you’ll find the most dramatic variation in the snowpack based on aspect.

High-elevations

Above 5,000ft you’ll encounter a snowpack similar to what you may have found around the 1st of March. Upper elevations have stayed mostly dry. The most sun-exposed slopes have surface crusts but have not seen much water or change to melt forms below the surface.

Problems

Wind Slabs

An icon showing Wind Slabs

We don’t expect significant snow from this storm but throw in a little wind, and you may find fresh wind slabs. The wind will affect the snow greatest at higher elevations, near exposed ridgelines, and in narrow mountain passes.  Keep your eyes open for signs that the wind drifted the snow into slightly deeper and firmer slabs. If you see textured snow surfaces, small snow drifts, or locally deeper accumulations be leery of nearby slopes greater than 35 degrees where you could trigger a small wind slab.  

Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 26th, 2019 11:00AM